Pyroelectric infrared detectors are used in many applications where background electronic noise can be a severe impediment to maximizing the performance of a system which utilizes these devices. The use of pyroelectric sensors using polarized crystals for detection of thermal radiation is well known. There have been many uses developed for such pyroelectric sensors. One important application is in the field of intruder alarms in which a pyroelectric sensor is arranged to detect the infrared radiation from a human being as an indication of an intruder. The invisible glow of materials permits pyroelectric infrared detectors to be used in detecting, monitoring, locating, counting and the like applications. Since every object is emitting infrared light, every object is a transmitter.
However, some pyroelectric infrared detectors can be falsely triggered by other sources of infrared radiation such as hot air from a heating system, sunlight, or equipment which may give off heat radiation. To discriminate between an element and such environmental effects, one method is to require movement of the source of heat. For this purpose, it is known to utilize at least two collocated sensors in a differential connection so as to produce zero net signal output when both sensors are irradiated and to produce multiple pulse outputs when a person walks past the device, as illustrated in U.S. Pat. No. 3,839,640 wherein two such sensors are connected in series opposition.
U.S. Pat. No. 4,441,023 discloses a compensated pyroelectric sensor having two separate but closely spaced lithium tantalate pyroelectric crystal detectors connected in parallel and poled such that output connections of opposite polarity are connected to each other. The dual sensor produces a much greater output voltage when one crystal is irradiated than would occur if the two detectors were connected series opposing.
It is also known in the prior art to connect two pyroelectric sensors in parallel opposition. Such connection will also reject signals produced by radiation common to both elements and has the advantage of a much higher sensitivity and output when only one sensor is energized. Such units also are free of the static build up problem. Typical of this type of device are the structures disclosed in U.S. Pat. Nos. 3,877,308 and 3,453,432 which disclose a pyroelectric radiation detector having at least two parallel opposed connected sensors.
Pyroelectric is the ability of certain materials to generate a temporary voltage when they are heated or cooled. The resulting polarization change gives rise to a voltage across the crystal. The crystal has a thermal time constant so it will thermalize to the ambient environment after a step input. The background noise can occur since pyroelectric infrared detectors require an impedance converting amplifier which is single ended wherein any electronic perturbation from an outside source such as line frequency interference or noise from an external but nearby oscillator which may be used to operate a microprocessor circuit or other sources of electronic noise can be coupled onto the detectors. Practical pyroelectrics contain either a JFET source follower in a voltage mode or a transimpedance amplifier in a current mode. The voltage mode circuit is known to generate the best signal to noise ratio. Other infrared detectors which do not require an impedance conversion circuit such as thermopiles are relatively low impedance devices are less sensitive to this problem.